Change field encode_name inspection

[Olegt0rr]

          `msgspec.inspect.type_info` is one way, but it's a bit overkill for this use case. And `__struct_encode_fields__` is technically non-public (and may be removed in the future). The best way to get this information is to use `msgspec.struct.fields` ([docs](https://jcristharif.com/msgspec/api.html#msgspec.structs.fields)). You're looking for the `encode_name` attribute on `FieldInfo` in the result:

In [3]: class Example(msgspec.Struct):
   ...:     user_id: int = msgspec.field(name="userId")
   ...: 

In [4]: msgspec.structs.fields(Example)
Out[4]: (FieldInfo(name='user_id', encode_name='userId', type=<class 'int'>, default=NODEFAULT, default_factory=NODEFAULT),)

Originally posted by @jcrist in jcrist/msgspec#672 (comment)